In the United States digital television signals broadcast over the air in accordance with the ATSC standard are transmitted using channel allocations similar to those used to broadcast analog television signals over the air in accordance with the NTSC standard. During the period of transition from broadcasting NTSC signals to broadcasting ATSC signals, television signal receivers capable of receiving signals broadcast according to each of these standards will be used. These television signal receivers may be complete television sets with viewscreens, set-top converters for NTSC receivers already in the field, television signal receivers for use with color television monitors, or television signal receivers which recover data for recording rather than presentation on a viewscreen. It is desirable that these NTSC/DTV receivers be capable of being automatically conditioned for operation in an NTSC reception mode responsive to the reception of an NTSC broadcast signal.
It is also likely that cablecasting of both NTSC signals and DTV signals using the same transmission channels will take place, although DTV cablecasting will not be done in accordance with the same ATSC standard used for terrestrial over-the-air broadcasting. At this time DTV cablecasting is done using quadrature-amplitude-modulation (QAM) signals rather than the vestigial sideband (VSB) signals advocated by the ATSC standard. Receivers both for terrestrial over-the-air broadcast DTV and for cable DTV have been proposed which use a common first detector for both modes of reception; and the two modes of reception have been distinguished in various ways, usually relying on features of the ATSC VSB signal for terrestrial over-the-air broadcast DTV that are not found in the QAM signal. If this is done, the distinction between the reception of an NTSC signal and a DTV cannot be predicated, at least not primarily, on the non-existence of distinctive features of the ATSC VSB signal. This is because those distinctive features are also absent during the reception of QAM DTV signals.
One can attempt to ascertain reception of an NTSC signal by detecting the presence of NTSC video carrier at the video detector of the NTSC receiver portion of the NTSC/DTV receiver. However, components near NTSC carrier frequency may occur as components of DTV signal or of wideband noise in a vacant transmission channel, resulting in erroneous indications of NTSC reception.
One can attempt to ascertain reception of an NTSC signal by detecting the presence of NTSC audio carrier. In this method also, erroneous indications of NTSC reception may result from detecting wideband noise in a vacant transmission channel.